Disc drives with the capability of recording onto compact discs (CDs) and/or digital versatile discs (DVDs) are tremendously popular. Writing on a disc is a very precise operation, necessitating tight control of the laser with constant power output. This is rendered somewhat problematic by the fact that a laser diode is a relatively unstable device with an efficiency that varies with time and temperature. Laser diodes are also susceptible to an aging process that affects their performance.
In DVD and CD recordable drives, the laser power delivered to the media in the write and read modes is controlled by a very low frequency power monitoring feedback loop. Based on the feedback, the current through the diode is appropriately regulated. The need for the feedback loop arises from the variation of the laser diode's efficiency with time and temperature. The event that is being sampled by the feedback loop is a laser light pulse with a width that is typically less than 10 nanoseconds. The bandwidth of the feedback loop is rather low, typically less than 100 kHz. Also, known as an automatic power control (APC), this feedback loop captures the amplitude of each level of drive operation (for example, read, erase, write, and anneal). Based on the current active drive operation mode, the appropriate control voltage is then fed back to the laser diode driver (LDD). Typically one channel will be needed for each level of drive operation mode.
FIG. 1 shows an existing implementation of the loop involve an APC circuit 102 that monitors a small fraction of the laser beam such as with optical feedback 104 via a partially deflecting mirror. The pulse-pattern output of the APC 102 is transmitted down a pickup head flex cable to the motherboard where a sample-and-hold circuit, such as A/D converter 100, captures the respective levels. Further digital signal processing at Digital Signal Processor (DSP) 108 ensures and control signals are fed back to the LLD 112. The fundamental speed limitation of this architecture lies in the response time of the APC/sample-and-hold combination and the need to transmit very fast rise time pluses down the flex.
Several problems are evident in the existing approaches, including temperature sensitivity, nonlinearity, and noise. All power is concentrated in small areas of the device. With several channels, the device will have several hot spots that generate thermal gradients and create thermal instabilities. Accordingly, the temperature of the chip will depend on the current operating mode. Another problem is that as the voltage rises, the diode behaves less like a conventional diode and more like an open-loop current mirror or a short-circuit. Nonlinear effects result, negatively impact the device's operation. Any noise that may be generated is multiplied by a large factor. This large amount of gain also produces inherent nonlinearities in the current transfer function. Current compliance problems result and may substantially interfere with operation of the device. Also, inevitably, the longer the flex, the more noise is introduced, and also the more cost.